Support for plain AVX with no FMA

Author: jmvalin

src/nnet.c

@@ -41,8 +41,11 @@
 
 #define SOFTMAX_HACK
 
-#ifdef __AVX2__
+#ifdef __AVX__
 #include <immintrin.h>
+
+
+#ifdef __AVX2__
 static __m256 exp8_approx(__m256 X)
 {
    const __m256 K0 = _mm256_set1_ps(0.99992522f);
@@ -65,7 +68,44 @@ static __m256 exp8_approx(__m256 X)
    Y = _mm256_castsi256_ps(_mm256_and_si256(mask, _mm256_add_epi32(I, _mm256_castps_si256(Y))));
    return Y;
 }
-
+#else
+#define _mm256_fmadd_ps(a,b,c) _mm256_add_ps(_mm256_mul_ps(a, b), c)
+#define _mm_fmadd_ps(a,b,c) _mm_add_ps(_mm_mul_ps(a, b), c)
+static __m128 exp4_approx(__m128 X)
+{
+   const __m128 K0 = _mm_set1_ps(0.99992522f);
+   const __m128 K1 = _mm_set1_ps(0.69583354f);
+   const __m128 K2 = _mm_set1_ps(0.22606716f);
+   const __m128 K3 = _mm_set1_ps(0.078024523f);
+   const __m128 log2_E = _mm_set1_ps(1.44269504);
+   const __m128 max_in = _mm_set1_ps(50.f);
+   const __m128 min_in = _mm_set1_ps(-50.f);
+   const __m128i mask = _mm_set1_epi32(0x7fffffff);
+   __m128 XF, Y;
+   __m128i I;
+   X = _mm_mul_ps(X, log2_E);
+   X = _mm_max_ps(min_in, _mm_min_ps(max_in, X));
+   XF = _mm_floor_ps(X);
+   I = _mm_cvtps_epi32(XF);
+   X = _mm_sub_ps(X, XF);
+   Y = _mm_fmadd_ps(_mm_fmadd_ps(_mm_fmadd_ps(K3, X, K2), X, K1), X, K0);
+   I = _mm_slli_epi32(I, 23);
+   Y = _mm_castsi128_ps(_mm_and_si128(mask, _mm_add_epi32(I, _mm_castps_si128(Y))));
+   return Y;
+}
+static __m256 exp8_approx(__m256 X)
+{
+   __m256 Y;
+   __m128 Xhi, Xlo, Yhi, Ylo;
+   Xhi = _mm256_extractf128_ps(X, 1);
+   Xlo = _mm256_extractf128_ps(X, 0);
+   Yhi = exp4_approx(Xhi);
+   Ylo = exp4_approx(Xlo);
+   Y = _mm256_insertf128_ps(_mm256_setzero_ps(), Yhi, 1);
+   Y = _mm256_insertf128_ps(Y, Ylo, 0);
+   return Y;
+}
+#endif
 
 static float celt_exp(float x)
 {